CMOS image sensors are used in a wide range of applications. In many applications, the sensor is operated with a so-called rolling shutter mode. If the exposure period needs to be reduced, the timing of the sensor is adapted so that only a sub-set of the total set of rows in the sensor array are integrating light during the image readout time. This sub-set of rows can be considered as a window which rolls over the focal plane array, hence the name ‘rolling shutter’.
Some applications, such as machine vision and motion analysis, demand a global shutter (also called a snapshot shutter) which allows the capture of all of the pixels of the sensor during the same time period. There are two main types of global shutters: a triggered global shutter and a pipelined global shutter. In a triggered global shutter the image must be read out before the next image can be captured. In a pipelined global shutter a new image can be captured during the readout of the image data from the previous image. Triggered global shutters are typically used in machine vision where an object needs to be inspected. Pipelined global shutters are typically used in motion analysis and high frame rate cameras. In a continuous recording mode, a pixel with a pipelined shutter is sensitive at all times.
Image sensors can be implemented using Charge Coupled Device (CCD) technology or Complementary Metal Oxide Semiconductor (CMOS) technology. An interline-transfer CCD device inherently allows pipelined global shutter operation. However, it is more difficult to implement a global shutter in CMOS image sensors. There have been several proposals for providing a global shutter in a CMOS image sensor. U.S. Pat. No. 7,224,389 shows a pipelined synchronous shutter pixel. The pixel comprises a photodiode, a reset transistor, a first buffer amplifier, a sample capacitor and a second buffer amplifier. The next image can be acquired during readout of an image, thus allowing pipelined shutter operation. The pixel does not allow cancelling non-uniformities caused by threshold voltage variations in the buffer amplifier or reset transistors in pipelined shutter operation. There is no possibility to obtain a reference level of the pixel during readout of the image, without destroying the signal on the photodiode, which will be capturing the next image in pipeline shutter operation.
The paper “A 600×600 pixel, 500 fps CMOS Image Sensor with a 4.4 μm Pinned Photodiode 5-Transistor Global Shutter Pixel”, I. Takayanagi, et al, proc. International Workshop on Image Sensors, Maine, June 2007, p. 287 describes a 5-transistor pixel which can perform a pipelined shutter operation and fixed pattern noise correction. U.S. Pat. No. 6,847,070 shows a 5-transistor pixel with the same topology. The pixel is shown in FIG. 1 and comprises a pinned photodiode, a transfer gate, a floating diffusion, a reset transistor, a source follower, a selection transistor and a separate anti-blooming transistor connected to the photodiode. The floating diffusion is used for storage of the signal during exposure of the next signal. A reference level can be read out by resetting the floating diffusion after the readout. The pixel thus allows fixed pattern noise correction and pipelined shutter operation. However, storing charges at the floating diffusion has several drawbacks. The floating diffusion is light sensitive, which means that the signal stored at the floating diffusion will be influenced by light collected during the storage time. Since the time that the signal is stored at the floating diffusion is larger at the last rows of the image than at the first rows, this can create a (light-dependent) gradient in the image, with a brighter area near the last rows of the image. A second problem is that the storage node is a surface junction which has a considerable leakage current. This leakage current will be added to the signal stored on the storage node, and thereby increase the noise on the sample. An additional anti-blooming transistor in the pixel is utilized to drain away excess charges, which might otherwise disturb the signal stored on the floating diffusion. The anti-blooming transistor can also be used to drain the photodiode during part of the readout time, when the required shutter time is lower than the frame readout time.
U.S. Pat. No. 7,286,174 describes a dual storage node pixel which is intended to store the signal level of a photosite recorded in each of two different frames, such as a high-speed imaging application where a scene is differently lit between two frames. A signal level of the photosite is transferred to one of the storage capacitors after each exposure. This signal is either transferred in the charge domain, in which case the charge is converted to voltage on the storage capacitor, or in the voltage domain, in which case the signal is converted to a voltage on the photosite.
Fixed pattern noise in CMOS pixels is largely caused by threshold voltage variations of the different transistors inside the pixel. The buffer amplifier (source follower) and also the reset transistor in the pixel will have variations in threshold voltage. Some reasons for the threshold voltage variations are local variations in dopant concentration in the transistor channel, oxide thickness, and dopant concentration of the gate. This threshold voltage variation results in a variable offset level of the pixel output signal. Usually, this offset variation is cancelled by measuring a reference level of the pixel which does not contain a photosignal, and subtracting this reference level from the measured signal level. To perform noise correction, the pixel must support measurement of this reference level.
The present invention seeks to provide a pixel, and a pixel array, which is capable of global shutter operation and which overcomes at least one of the problems of existing pixels.